Although i do think that the SR thing for melee-oriented classes is necessary for balance between casters and meleers, i must say, that fluff-wise, it doesn't really make much sense.

Magic isn't innate to all creatures and classes, some have more contact with it, or were born in contact with it, some trained to resist their effects. A lvl 1 commoner is a possibility of a fighter, or a cleric, or a wizard. With dedication, they can overcome their common lives and become adventurer's themselves. At which point in that transition do they gain that SR? Unless you are saying that no creature is 100% vulnerable to magic, that doesn't really make much sense.

I reason and envision SR as being somewhat of a supernatural ability (Not in the strict sense of the rules), something you are just born with or you otherwise gain by supernatural means. That's why some races (like the Drow) have SR as a racial ability. I don't see a reason beyond balancing the rules for a fighter to have SR, or a barbarian for that matter. Sure, a monk can and should have SR, because they spend time perfecting their minds and their bodies to resist all kinds of hazards (that's why they have all those special abilities on their class progression list), and even the Monks only gain their SR at a later level, at a later stage of their training for perfection.

So, while i don't criticize your fix(on the contrary, i really think it brings balance), i think it overshadows a bit of the magic as being a "mysterious" and "wondrous" thing, and just some other form of attack. Think of it as kind of DR, you only gain DR if you wear special armor, or if you have thick skin. So you should only gain SR if you somehow are more resilient to magic than normal.

Although i think it's overly more complicated, fixing the spells and the casters is more fluff kind than the SR fix, but it's just my thoughts. If someone on my gaming table was using a wizard in a munchkin way, i would definitely house-rule this to minimize the hurting.

All in all, i think the other class fixes are just perfect. (Except the ranger, but that's because i simply can't think of a ranger using magic, so i prefer to use a non-magic variant that gains the animal companion at level 1)

Just my thoughts.

please don't hurt me